Sihler, A.
New Comparative Grammar of Greek and Latin.
Oxford University Press.
2008.

justo
(Adjective)
"just," "fair"
12th cent. From Latin iustus 'id.'
From Proto-Italic *jowes-to- 'id.' Potentially a somewhat late adjective derived from *jowes- "oath." For a continued etymology, see jurar.
The word is the origin of a number of surnames, Justo, Justa, Justal, Juste, Justos, Justes. Also the origin of Justel, a small village in Leon.
Indo-European